1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device, and more particularly to a method for forming a field oxide film of a semiconductor device so as to electrically isolate device elements in the semiconductor device.
2. Description of the Prior Art
Generally, an LOCOS (Local Oxidation of Silicon) technique is very well-known as a device isolating method of a semiconductor device in the art. During the LOCOS process, a Bird's beak is always generated. The Bird's beak means the shape of an oxide material that has penetrated into an active region of a semiconductor device. Because of such a Bird's beak, the width of an active region in the device is substantially shorten in comparison to a pattern of a mask for forming a field oxide film. Occurrence of such a Bird's beak has a seriously bad effect on the fabrication of an ULSI (ultra-large scale integrated) device required for integration of 64 mega or more.
Recently, to reduce a size of a Bird's beak, a PBL (polysilicon buffered LOCOS) method, which uses a polysilicon having a characteristic of a further fast thermal-oxidation in comparison to a single crystal, has been developed.
FIG. 1 shows a partial structure of a semiconductor device which is fabricated by the above-described PBL method. In FIG. 1, a reference numeral 15 indicates a polysilicon layer formed between field oxide regions 19, 19 and located on a silicon substrate 11 with a pad oxide film 13 interposed therebetween. While performing a PBL process, since an oxidation speed of the polysilicon 15 is faster than that of the silicon substrate 11, the polysilicon 15 serves as a protective film to prevent penetration of oxygen into an active region of the silicon substrate 11, whereby enlargement of a Bird's beak can be obstructed.
However, a nitride film 17 formed on the polysilicon 15 is strained due to a stress occurring upon oxidation of the polysilicon 15 being performing, and thus oxygen is penetrated through a gap between the strained nitride film 17 and the polysilicon 15. As a result, a Bird's beak having a width BB is produced as shown in FIG. 1.
FIG. 2 shows another structure of a portion of a semiconductor device which is fabricated by an enhanced PBL method. Similarly, the structure of FIG. 2 has a polysilicon layer 25 between field oxide regions 29, 29 and located on a silicon substrate 21 with a pad oxide film 23 interposed therebetween. To prevent oxygen from being penetrated through a side of the polysilicon 25 and reduce a width of a Bird's beak, after formation of a side nitride film 28 an oxidation is performed. However, as in the PBL method shown in FIG. 1, since it is impossible to completely prevent oxygen from being penetrated through an interface between the side nitride film 28 and the substrate 21, a stress occurs under the side nitride film 28 during oxidation. As a result, a gap in the interface is wide, and thus a Bird's beak is produced by following oxidation, as shown by BB in FIG. 2.
FIGS. 3A and 3B show the structure of a portion of a semiconductor device which is fabricated by a most recently developed PBL method, wherein FIG. 3A is a cross-sectional view of the partial structure of the semiconductor device before performing a thermal oxidation process and FIG. 3B is a cross-sectional view of the partial structure after performing a thermal oxidation process.
As shown in FIGS. 3A and 3B, on a silicon substrate 31 first and second pad oxide films 33, 34 are sequentially formed, and then the second pad oxide film 34 is over-etched by using a patterned nitride film 37 thereon as a mask to form a side groove. Next, a polysilicon layer 35 is deposited over the nitride film 37 including the side groove and is thermal-oxidized to prevent the nitride film 37 from being strained and prevent oxygen from penetrating through the side groove during oxidation.
As described above, to minimize a size of a Bird's beak, prior art methods have been proposed in the art, but such occurrence of a Bird's beak can not be prevented by the prior art methods developed up to now.